This application is based on French Patent Application No. 01 03 679 filed Mar. 19, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The present invention relates to a method of controlling the discharging of a battery including at least two secondary storage cells, and in particular to a method of detecting the end of discharging. The method is more particularly intended to be applied to an alkaline electrolyte secondary storage cell battery, especially a nickel/metal hydride battery.
2. Description of the Prior Art
The term xe2x80x9cindustrialxe2x80x9d applies to a storage cell which has a high capacity, meaning a capacity greater than 10 Ah, and generally from 50 Ah to 200 Ah. Industrial storage cells usually have a prism-shaped plastics material container containing plane electrodes. Their parallelepipedal geometry and the nature of their tank means that they cannot resist high pressures, and their maximum internal pressure is of the order of 1 bar to 2 bar. xe2x80x9cMaintenance freexe2x80x9d industrial storage cells and xe2x80x9csealedxe2x80x9d industrial storage cells have the advantage that they do not necessitate topping up of the level of the aqueous electrolyte at any time during their use, unlike xe2x80x9cstandard unsealedxe2x80x9d industrial storage cells or xe2x80x9creduced maintenance unsealedxe2x80x9d industrial storage cells.
Batteries of such storage cells are used in the aeronautical, rail, road and stationary fields. They are intended, among other things, for electric vehicle traction. In this application, the required service life is approximately 1 500 charge/discharge cycles over an estimated period of 10 years. A service life of this length can be achieved only if premature deterioration of the storage cells is avoided. In particular, discharging must be strictly controlled to avoid any inversion or overdischarging of any of the cells.
For standard nickel-cadmium unsealed industrial storage cell batteries, overdischarging has little influence on the service life of the storage cells because the consumption of water is periodically compensated by topping up as a maintenance operation. The end of discharging is managed by way of the total voltage of the battery. Discharging is stopped when the battery voltage reaches a particular value.
There is a decrease in power towards the end of discharging. To make this reduction in power more gradual, the storage cells are grouped together (in groups of 5 to 10 storage cells, for example) and the voltages of the groups are compared with each other: the greater the difference, the nearer the end of discharging. Discharging is stopped when the voltage difference xcex94V between the voltages of two of the groups of storage cells reaches a prescribed value.
An improvement to this latter method consists in weighting the voltage difference xcex94V by the average value Vavg of the groups of storage cells to obtain a weighted value xcex94V/V which is compared to a specified criterion.
Maintenance-free and sealed industrial storage cells, in particular nickel/metal hydride cells, are more fragile with regard to overdischarging, among other things because of the generation of gas, the limited quantity of electrolyte and deterioration of the active materials as a result of inversion. If the same method is applied, it is necessary to compare the voltage of modules formed from grouped storage cells, but discharging must be stopped when the difference between the voltages of the modules reaches a sufficiently low threshold to prevent overdischarging. It may nevertheless happen that the threshold value is reached even though the battery is not yet at the end of discharging. This phenomenon can be due to the variation of the internal resistance between the storage cells (differential aging, replacement of a storage cell, etc.), the variation in the temperature inside the battery, or variation in the state of charge between the modules.
An object of the present invention is to propose a method of controlling the first phase of discharging a storage cell battery when there is a variation in internal resistance between the storage cells.
The present invention provides a method of controlling the discharging of a battery including a plurality of modules each consisting of at least one secondary storage cell, which method includes the following steps:
(a) a voltage V(tn) of each module and a discharge current I(tn) are measured synchronously at time tn,
(b) the internal resistance IR(tn) of each module is calculated as follows: IR(tn)=V(tn)/I(tn),
(c) a voltage V(tn+1) of each module and a discharge current I(tn+1) are measured synchronously at time tn+1,
(d) the internal resistance IR(tn+1) of each module is calculated as follows: IR(tn+1)=V(tn+1)/I(tn+1),
(e) the slope of the variation in the internal resistance IRS of each module is calculated between times tn and tn+1 as follows: IRS=[IR(tn+1)xe2x88x92IR(t)]/I(tn),
(f) the maximum value IRSmax and the average value IRSavg of the slopes of the variation in the internal resistance of all the modules are calculated,
(g) a difference DIRS is calculated as follows: DIRS=IRSmaxxe2x88x92IRSavg,
(h) the difference DIRS is compared to a criterion K determined experimentally, and
(i) the first phase of discharging is stopped when DIRS is greater than or equal to K, following correction of the rate of change dI/dt of the discharging current I(t).
It is seen that the end of the first phase of discharging of the battery is determined by the module having the greatest internal resistance slope.
The method according to the invention makes the end of discharging more progressive by introducing a second phase of discharging and prevents the inversion of a module at the end of discharging in spite of the variation in internal resistance between the modules.
In the particular case in which the battery is made up of two modules each consisting of at least one secondary storage cell, the method includes the following steps:
(a) a voltage V1(tn) for the module 1, a voltage V2(tn) for the module 2, and a discharge current I(tn) are measured synchronously at time tn,
(b) internal resistances IR1(tn) and IR2(tn) are calculated as follows: IR1(tn)=V1i(tn)/I(tn) and IR2(tn)=V2(tn)/I(tn),
(c) a voltage V1(tn+1) for the module 1, a voltage V2(tn+1) for the module 2, and a discharge current I (tn+1) are measured synchronously at time tn+1,
(d) internal resistances IR1(tn+1) and IR2 (tn+1) are calculated as follows: IR1(tn+1)=V1i (tn+1)/I(tn+1) and IR2 (tn+1)=V2 (tn+1)/I(tn+1),
(e) the slopes of the variation in the internal resistances IRS1, and IRS2 between times tn and tn+1 are calculated as follows: IRS1=[IR1(tn+1)xe2x88x92IR1(tn)]/(tn+1xe2x88x92tn) and IRS2=[IR2(tn+1)xe2x88x92IR2 (tn)]/(tn+1xe2x88x92tn),
(f) a difference DIRS is calculated between the slopes of the internal resistances of the modules 1 and 2 as follows: DIRS=IRS1xe2x88x92IRS2,
(g) the difference DIRS is compared to a criterion K determined experimentally, and
(h) the first charging phase is stopped if DIRS is greater than or equal to K, after correcting the rate of charge dl/dt of the discharge current I(t).
This correction of the slope dl/dt of DIRS is rendered necessary by the dynamic aspect of electric vehicle operation, in which acceleration and deceleration phases in which the current is reversed alternate. Various laws can be envisaged for correcting the difference DIRS between the internal resistance slopes of the modules as a function of the slope dl/dt of the curve of the variation in time of the current I (t). In a preferred embodiment, DIRS is a linear function of the variation dl/dt of the discharge current I (t) in time.
The second phase of discharging can be either a complete halting of discharging or a progressive reduction of the discharging current I(t). The discharging current I(t) during the first phase of discharging is preferably at least equal to 100 A.
A further advantage of the present invention is that the method according to the invention generates intermediate alerts leading to a reduction of the discharging current in order to authorize a progressive end of discharging.
The following notation is used in the following description:
I (t): the value of the discharging current of the battery at time t, expressed in amperes;
Vi(t): the voltage of a module i of the battery at time t, expressed in volts;
Vavg: the average of the module voltages, expressed in volts;
Vmin: the lowest module voltage, expressed in volts;
xcex94V=Vavgxe2x88x92Vmin: the difference between the average value Vavg and the lowest value Vmin of the module voltages, expressed in volts;
xcex94V/V=(Vavgxe2x88x92Vmin)/Vavg: the difference between the average value Vavg and the lowest value Vmin of the module voltages, weighted by the average voltage Vavg of the modules constituting the battery;
IRi(t): the internal resistance of a module i at time t, expressed in ohms;
IRSi: the slope of the variation in the internal resistance of the module i, expressed in ohms per second;
IRSmax: the highest internal resistance variation slope IRS of the modules constituting the battery, expressed in ohms per second;
IRSavg: the average of the internal resistance variation slopes of all the modules constituting the battery, expressed in ohms per second;
DIRS: the difference between the maximum slope IRSmax and the average slope IRSavg of the internal resistances of the modules constituting the battery, expressed in ohms per second, which depends on the variation in time of the discharging current; and
K: the comparison value of the criterion DIRS for determining the end of the first discharging phase, which is a constant determined by experiment and expressed in ohms per second.
There follows a description of embodiments of the invention, given with reference to the accompanying drawings.